This research is concerned with the physiological and genetic mechanisms and evolutionary significance of litter size variation in the cotton rat, Sigmodon hispidus. Two populations have been identified in which the rats differ significantly in number of young produced per litter. The first objective is to establish whether this difference is a proximate response to an environmental difference or whether it represents an evolved difference in life history of the rats. During the first 9 months of the project animals have been collected from the two populations and experiments have been set up to determine the nature of the genetic component (if important) and to assess the interaction of this component with a potentially important environmental factor, food quality. A second objective is to compare the investment of several resources by the mother rats in offspring production and to document the effects of resource shortage on size and number of offspring produced and risk to the parent.